Chemical Stabilization of Hexanal Molecules by Inclusion as Guests of

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Chemical Stabilization of Hexanal Molecules by Inclusion as Guests of Nanoporous-Crystalline Syndiotactic Polystyrene Crystals Paola Rizzo,* Antonietta Cozzolino, and Gaetano Guerra*

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Dipartimento di Chimica e Biologia, INSTM Research Unit, Università di Salerno, Via Giovanni Paolo II, 84084 Fisciano (SA), Italy ABSTRACT: Chemical stabilization of air-unstable molecules, by their inclusion as guest of nanoporous-crystalline phases of syndiotactic polystyrene (s-PS), is for the first time revealed. FTIR analyses show that antimicrobial hexanal molecules are chemically stable for months when guest of s-PS cocrystalline phases, while they easily oxidize to hexanoic acid when they are simply dissolved in polymer amorphous phases. Formation of s-PS/hexanal cocrystalline forms is clearly demonstrated by polarized FTIR spectra and by X-ray diffraction patterns. Chemical stabilization is observed for hexanal molecules being guest of both nanoporous-crystalline forms of s-PS, i.e., for guest molecules isolated in the cavities of δ form or aligned along the channels of ε form. Films exhibiting s-PS/hexanal clathrate forms are able not only to chemically stabilize the antimicrobial molecules but also to give their long-term release (with a diffusivity close to 2 × 10−14 cm2 s−1). Possible mechanisms of chemical stabilization by clathrate formation are discussed. has a positive effect on their shelf life.56−58 In fact, it has a significant inhibitory effect against pathogenic microorganisms often isolated from raw materials (E. coli, S. enteritidis, and L. monocytogenes),59−61 and it also shows antifungal activity against A. niger.62 Hexanal, however, is an unstable molecule which suffers the problem of poor stability to oxidation already in air at room temperature,63−65 so it must be generally preserved at low temperatures (in the range 2−8 °C). In the present contribution, hexanal chemical stability in different polymeric environments is studied by FTIR analysis, while the possible formation of a s-PS/hexanal cocrystalline forms is studied by X-ray diffraction and polarized FTIR analyses.

1. INTRODUCTION The inclusion of guest molecules in host crystalline phases can lead to remarkable changes of physical and chemical properties.1−5 In fact, in suitable host environments, guest molecules can reduce or increase their chemical stability.6,7 For many technologically relevant fields, mainly for those involving long-term active guest release, the improvement of the chemical stability of the guest molecules can be highly beneficial. A controlled release of active guests from cocrystalline polymer phases has been described in the literature.8−12 These cocrystalline phases are obtained by absorption of active molecules in nanoporous-crystalline phases (exhibiting a density definitely lower than for the corresponding amorphous phases) and are available for two commercial polymers: syndiotactic polystyrene (s-PS)13−16 and poly(2,6-dimethyl1,4-phenylene) oxide.17−19 The thermoplastic nature of these materials allows their easy processing to products suitable for several applications like films,20−33 membranes,34 and foams as well as their easy recycling. As a consequence, a number of applications in the fields of pollution remediation,35−40 gas sorption and separation,41−43 sensorics,44−46 and catalysis47−49 for these inexpensive and reusable materials have been proposed. In this paper, the ability of nanoporous-crystalline polymer films to induce chemical stability to their guest molecules is for the first time revealed. Chemical stabilization is often deserved for antimicrobials, which are widely used for many applications50,51 including food protection.52,53 In particular, hexanal, an aldehyde present in fruits and vegetables, has toxic action toward parasites that attack the fruit and cause it to fall.54,55 It has been also shown that the addition of this compound in food packaging systems containing fresh fruits © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. Hexanal is an aldehyde with Tm = −56 °C and Tb = 131 °C. All used solvents were supplied by Sigma-Aldrich. Isotactic polypropylene MOPLEN PP310D produced by Lyondell Basell was used. Films, obtained by a bubble extrusion process using a melt temperature of 190 °C, present a thickness of 30 μm and exhibit the α crystalline form with a degree of crystallinity close to 45%. Syndiotactic polystyrene, with the trademark Xarec 300ZC, was provided by Idemitsu. The content of syndiotactic triads, evaluated by 13 C nuclear magnetic resonance, is >98%. The mass average molar mass, determined by gel permeation chromatography (GPC) in trichlorobenzene at 145 °C, is Mw = 140000 g mol−1, and the polydispersity index is Mw/Mn = 2.0. Axially oriented s-PS films were obtained by stretching of amorphous films, with a dynamometer INSTRON 4301, at 105 °C, with draw ratio 300% and strain rate of 0.16 s−1. Received: October 9, 2018 Revised: February 1, 2019

A

DOI: 10.1021/acs.macromol.8b02168 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. FTIR transmission spectra, in the wavenumber range 1800−1650 cm−1, for polymer films after sorption of hexanal and subsequent desorption at room temperature: (A) i-PP, (B) γ form s-PS, (C) ε form s-PS, and (D) δ form s-PS. Different curves correspond to the indicated desorption times. Black curves in the bottom of the figures show the spectra of untreated polymer films. and with a resolution of 2.0 cm−1. The frequency scale was internally calibrated to 0.01 cm−1 using a He−Ne laser. To reduce the noise, 32 scans were signal averaged. Infrared polarized spectra were obtained by using a SPECAC 12000 wire grid polarizer. An accurate evaluation of the degree of crystallinity for all s-PS samples was performed by a FTIR spectral subtraction procedure, as described in detail in a previous report.69 The degree of crystallinity of all s-PS samples is in the range 35−40%. The intensity of FTIR guest peaks was used to evaluate the content of the guest molecules in the films by previous calibration using thermogravimetric measurements. Thermogravimetric measurements (TGA) were performed with a TG 209 F1 Netzsch in the temperature range 25−350 °C and at a scanning rate of 10 °C/min. The degree of orientation of the chain axes of the host crystalline helices with respect to the stretching direction has been evaluated by using the axial orientation factor:

Films with the nanoporous-crystalline δ form were obtained from axially oriented s-PS films by exposure to dichloromethane vapor at room temperature for 1 night and by guest removal by immersion in acetonitrile for 3 h. Films with the dense γ form were obtained by annealing of δ form films at 130 °C for 1 h.66−68 Films with the nanoporous-crystalline ε form were obtained by immersions of the γ form films for 1 h in chloroform and then in acetonitrile. δ, γ, and ε form films used for WAXD, FTIR, and polarized FTIR measurements are axially oriented and have a thickness of nearly 50 μm. 2.2. Techniques and Methods. An automatic Bruker diffractometer was used to get wide-angle X-ray diffraction patterns (with nickel filtered Cu Kα radiation). A Vertex 70 Bruker spectrometer equipped with deuterated triglycine sulfate (DTGS) detector and a Ge/KBr beam splitter was used to get infrared spectra in the wavenumber range 400−4000 cm−1 B

DOI: 10.1021/acs.macromol.8b02168 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. FTIR transmission spectra in the wavenumber range 3600−400 cm−1 of a ε (A) and a δ (B) form film, after immersion in liquid hexanal for 3 h and different desorption times up to 50 days. Asterisks indicate decreasing vibrational peaks of hexanal molecules. fh,IR = S h

2 cot2 θ + 2 2 cot2 θ − 1

The molecular volume Vm has been calculated according to ref 71 from the ratio between their molecular mass (M) and density (ρ) times NA, i.e., Avogadro’s number (6.02 × 1023 molecules/mol):

(1)

where θ is the angle between the chain axis and the transition moment vector of the vibrational mode and Sh is the order parameter of the host crystalline phase as reported in the following formula:

Sh =

R−1 R+2

Vm =

R−1 R+2

(4)

Based on the experimental values of the guest order parameter Sg, the guest fraction in the crystalline phase (xg,cr) can be evaluated by using the following formula:72

(2)

R = A∥/A⊥ is the dichroic ratio, and A∥ and A⊥ are the measured absorbance for polarization plane parallel and perpendicular to the draw direction, respectively. The dichroic ratio of the 572 cm−1 infrared peak, characterized by transition moment vector parallel to the chain axes (θ = 0°),70 was used to evaluate the axial orientation factor f h,IR for the nanoporous and clathrate phases of s-PS samples. The orientation factor of the used axially stretched δ and ε form films is f h,IR = 0.93 and f h,IR = 0.96, respectively. It is worth recalling that this orientation factor is equal to 1 for perfect alignment, 0 for random orientation, and −0.5 for perpendicular alignment. The polarized spectra of the clathrate films also allow an analogous evaluation of the degree of orientation of guest molecules with respect to the host stretching direction. To this purpose the highly dichroic and isolated carbonyl peak of hexanal guest molecules absorbed into δ and ε s-PS films (located at 1726 cm−1) has been considered, and the guest order parameter Sg has been evaluated: Sg =

M ρNA

Sg = (1 − xg,cr)Sg am + xg,crSg cr am

(5)

Sgcr

are the guest order parameter in the amorphous where Sg and and in the crystalline phase, respectively. We have evaluated xg,cr from the simplified equation

Sg = xg,crSg cr

(6)

where it is assumed absence of orientation of the guest molecules in the amorphous phase (Sgcr ≫ Sgam ∼ 0). Sgcr is assumed equal to the Sg plateau value, which is reached after long desorption times.

3. RESULTS AND DISCUSSION 3.1. FTIR Spectra. An FTIR spectrum of an isotactic polypropylene (i-PP) film, after 2 h of immersion in liquid hexanal at room temperature, in the spectral region 1800− 1650 cm−1 which is suitable to evaluate the amount of absorbed hexanal, is reported in Figure 1A. Spectra corresponding to different desorption times at room temperature (from 2 up to 120 min) are also reported in Figure 1A. The amount of absorbed hexanal, as evaluated by the intensity of the carbonyl stretching peak located at 1733 cm−1, is close

(3)

where R = A∥/A⊥ is the dichroic ratio, with A∥ and A⊥ being the measured absorbance of the peak at 1726 cm−1 for polarization plane parallel and perpendicular to the draw direction, respectively. C

DOI: 10.1021/acs.macromol.8b02168 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules to 3 wt % and rapidly decreases, becoming negligible after 2 h of room temperature desorption. The FTIR spectra of Figure 1A also show the presence of a carbonyl peak at 1713 cm−1, indicating the presence of hexanoic acid, as oxidation product of hexanal, already after the absorption process. It is worth adding that for the iPP film, beside the fast hexanal desorption, also a slower hexanoic acid desorption is observed. FTIR spectra in the same carbonyl region for a γ form s-PS film, i.e., of a semicrystalline film exhibiting a dense crystalline γ phase, after 12 h of absorption of hexanal (which leaves unaltered γ phase and degree of crystallinity) and subsequent room temperature desorption for nearly 7 weeks, are reported in Figure 1B. The amount of absorbed hexanal is again close to 3 wt % and only slowly decreases, with a still detectable residual content after 4 months of room temperature desorption. The FTIR spectra of Figure 1B as a consequence of hexanal desorption also reveals a carbonyl peak at 1709 cm−1, indicating the presence of hexanoic acid. It is also worth adding that the intensity of the carbonyl peak of the acid remains nearly unaltered with desorption time, clearly indicating that desorption of hexanoic acid from s-PS, for the considered times, is nearly negligible. FTIR spectra in the same carbonyl region for ε form14 and δ form70 s-PS films (Figures 1C and 1D, respectively), i.e., of semicrystalline films exhibiting nanoporous crystalline phases that can form clathrate structures with suitable low-molecularmass guest molecules, show a completely different behavior. In fact, after equilibrium sorption of liquid hexanal, the hexanal content is much higher (not far from 15 wt %). More relevantly, the carbonyl peak of hexanoic acid is barely detectable for ε form (Figure 1C) and δ form (Figure 1D) films, even after 2 months of room temperature desorption. Moreover, the whole FTIR spectra of ε form and δ form (Figure 2) s-PS films, even after long-term desorption, do not show vibrational peaks different from those of the host polymer and of the hexanal guest. This clearly indicates the occurrence of long-term stability of hexanal, when a guest of the nanoporous-crystalline ε and δ forms of s-PS. It is worth noting that hexanal maintains an analogous longterm stability also when occupying only a small fraction of the crystalline cavities. In particular, tests have been conducted by absorption hexanal from 1000 ppm aqueous solutions for 12 h, with heavily reduced hexanal sorption (≈1%), which corresponds to a fraction of filled crystalline cavities lower than 0.1. In these experiments, hexanal molecules have been included in a crystalline phase being mainly nanoporous and hence exhibiting an oxygen solubility at least 10 times higher with respect to the corresponding amorphous phase.73 For the four considered polymer films (i-PP, γ form s-PS, ε form s-PS, and δ form-PS), hexanal desorption kinetics, as evaluated by FTIR spectra like those of Figure 1, are compared in Figure 3. For the nanoporous crystalline s-PS (ε and δ form) films, the much higher absorption capacity and the only partial desorption of hexanal, with a residual hexanal content after 20 days of desorption higher than 6 wt %, are shown. This clearly suggests the inclusion of hexanal molecules in both crystalline phases, with formation of the corresponding s-PS/hexanal ε and δ cocrystalline (clathrate) forms. Many reports on desorption kinetics of guest molecules from sPS films have shown a Fickian behavior for both δ9,73,74 and ε74 nanoporous-crystalline phases. Hexanal desorption data at room temperature are reported versus the square root of desorption time divided by film thickness (√t/L) for δ and ε

Figure 3. Desorption kinetics at room temperature of hexanal from different polymer films: (■) δ form s-PS; (◆) ε form s-PS; (○) γform s-PS; (△) i-PP.

form films in Figures 4A and 4B (left scale), respectively. By considering hexanal desorption times longer than 20 days, i.e., when desorption occurs essentially only from the crystalline phase, a diffusivity close to 2 × 10−14 cm2/s for both nanoporous crystalline phases is obtained (dashed lines in Figure 4). This value is similar to that one obtained for

Figure 4. Hexanal desorption kinetics at room temperature (□) from ε-form (A) and δ-form (B) s-PS film versus the square root of desorption time divided by film thickness (√t/L). The right scale shows the guest order parameter Sg [Sg = (R − 1)/(R + 2)] (●), as calculated for the hexanal peak at 1726 cm−1. D

DOI: 10.1021/acs.macromol.8b02168 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. Polarized FTIR transmission spectra in the wavenumber range 3600−1250 cm−1 for polarization plane parallel (thin blue lines) and perpendicular (thick red lines) to the stretching direction of axially oriented s-PS films: (a) ε form; (b) ε form with 6.8 wt % of hexanal; (c) δ form; (d) δ form with 6.2 wt % of hexanal. Dichroic guest peaks are labeled with symbols (||) and (⊥) if their transition moment vectors are roughly parallel and perpendicular to the stretching direction, respectively. The inset figure enlarges the spectral range 3550−3400 cm−1 and shows the dichroism of a hexanal band at 3438 cm−1.

Figure 6. Hexanal fraction in the crystalline phase (xg) (A) and hexanal content (% w/w) in the whole film (filled circles) in its δ crystalline (empty circles) and amorphous (crossed circles) phases (B) as a function of desorption time.

carvacrol guest molecules desorption10 from δ crystalline phase of s-PS (≈4 × 10−14 cm2/s). 3.2. Polarized FTIR Spectra. The formation of s-PS/ hexanal cocrystalline forms is confirmed by polarized FTIR spectra of axially oriented ε form and δ form s-PS films, which

are reported in Figures 5a,b and 5c,d, respectively. In particular, spectra labeled as “a” and “c” have been collected before hexanal uptake while spectra labeled as “b” and “d” have been collected after hexanal uptake and partial desorption (hexanal content 6.8 and 6.2 wt %, respectively). The E

DOI: 10.1021/acs.macromol.8b02168 Macromolecules XXXX, XXX, XXX−XXX

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Figure 7. X-ray diffraction patterns of s-PS films exhibiting (A) nanoporous-crystalline ε form, (A′) sPS/hexanal ε clathrate form, (B) triclinic nanoporous-crystalline δ form, and (B′) sPS/hexanal monoclinic δ clathrate form. For ε form films, peaks due to a minor amount of γ form are labeled.

absorbance peaks of the s(2/1)2 helical polymer chains70,75(i.e., polymer chains of the host crystalline phase), like e.g. those at 1354 and 1277 cm−1, are highly dichroic (Figure 5), and high degrees of axial orientation have been evaluated (f h,IR ≈ 0.96 for ε form film and f h,IR ≈ 0.93 for δ form film). Most guest peaks are also dichroic and are labeled by (||) and (⊥), if higher absorbance is observed for spectra with polarization plane parallel and perpendicular with respect to the film stretching direction, respectively. Additional relevant information coming from the spectra of Figure 5 is the opposite signs of dichroism of the intense carbonyl stretching bandat 1726 cm−1, which is (||) or (⊥) for hexanal molecules being guest of ε and δ form films, respectively. The same phenomenon occurs for the definitely less intense peaks at 2955 and 2872 cm−1. A dichroism of opposite sign has been often observed for vibrational peaks of a same guest of ε and δ form films, and it is due to different guest orientations with respect to the helical chain axes.76−78 The variation of the dichroic ratio, as evaluated for the hexanal carbonyl band at 1726 cm−1, is shown as a function of the desorption time, in Figures 4A and 4B (right scale) for ε and δ form films, respectively. It is apparent that in both cases the absolute value of dichroism increases with guest desorption, with dichroism becoming progressively more positive and more negative for ε and δ form films, respectively. This phenomenon, as well established in the literature,70,72 is a clear evidence of guest inclusion in the crystalline cavities of sPS. In this framework, the increase of absolute values of guest dichroism associated with its partial desorption is rationalized by a faster desorption of molecules simply absorbed in the amorphous phase rather than of those guests of crystalline cavities.72 On the basis of the method described in ref 72, it is possible to conclude that after ≈20 days the hexanal concentration in the polymer amorphous phase is nearly negligible, for both ε and δ form films. The fraction of the guest molecules in the δ crystalline phase, based on the guest order parameters Sg shown in Figure 4B, has been evaluated and plotted in Figure 6A. The hexanal content in crystalline and amorphous phases is plotted in Figure 6B as empty and crossed circles, respectively. The data of Figure 6B show that for short times the hexanal desorption happens mainly from the amorphous phase, whereas the hexanal concentration in the crystalline phase remains

essentially unaltered. Analogous behavior is also observed for hexanal desorption from ε crystalline phase. 3.3. X-ray Diffraction. Formation of s-PS/hexanal ε clathrate and δ clathrate phases of s-PS is also clearly confirmed by X-ray diffraction characterization of the films. In particular, X-ray diffraction patterns of axially oriented ε form and δ form s-PS films (before and after 3 h of immersion in hexanal followed by 1 h of desorption at room temperature) are shown in Figures 7A, and 7B, respectively. The starting s-PS ε form film shows typical diffraction peaks of the orthorhombic form,14 together with two shoulders indicating the presence of a minor amount (